1. Field of the Invention
This invention relates to a numerical control method and, more particularly, to a numerical control method in an NC apparatus having a C-axis control function for positionally controlling a spindle in the direction of rotation.
2. Description of the Related Art
In a machine tool having a spindle motor, there are cases where a blank is subjected to predetermined machining while positional control is performed by a spindle motor in the same manner as with a servomotor. In a lathe, for example, a blank is subjected to drilling or grooving by positionally controlling a spindle, namely the blank, in the direction of rotation (C-axis direction), or the blank is subjected to machining such as helical grooving by simultaneously transporting a cutter while the blank is being rotated in the C-axis direction by numerical control.
It is difficult to make the servo-loop gain or acceleration/deceleration time constant in the spindle motor control system agree with the servo-loop gain or acceleration/deceleration time constant in the servomotor (feed motor) control system, and these values usually are different. Consequently, in a case where the amounts of delay in the motors differ from each other and C-axis control is performed at the same time as control along the other feed axis, the actual path of movement becomes displaced from the command path of movement owing to the disparity in the amounts of delay.
Accordingly, it has been contemplated, solely for cases where a changeover is made from velocity control in which a spindle is rotated at a fixed velocity to C-axis control, to manually set the characteristic values (acceleration/deceleration constant, servo-loop gain, etc.) of feed-motor axis control so as to agree with those of the spindle motor, and restore the characteristic values of spindle control to their original values at the end of C-axis control. However, this approach is troublesome since the manual changeover must be made manually whenever control is changed over.
Accordingly, an object of the present invention is to provide a numerical control method in which axis control characteristic values of a spindle motor and servomotor can be made to agree and then be restored to original values automatically.